Re: [RFC PATCH -RT] epoll: Fix eventpoll read-lock not writer-fair in PREEMPT_RT

On 2021-08-26 22:36:04 [+0206], John Ogness wrote:

On 2021-08-26, Sebastian Andrzej Siewior [off-list ref] wrote:

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On 2021-08-25 15:27:54 [+0200], Frederic Weisbecker wrote:

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I guess making readlock writer fair is still not the plan so all I can
propose is to make that rwlock build-conditional.

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It is writer fair in a sense that once a writer attempts to acquire
the lock no new reader are allowed in.

What you want is that the writer pi-boosts each reader which is what
is not done (multi reader boost). Long ago there was an attempt to
make this happen (I think with rwsem) but it turned out to be
problematic.  There was a workaround by only allowing one reader and
doing PI as usual.

This patch is essentially forcing that exact workaround for eventpoll.

Frederic ended the mail with "readlock is not writer fair" so I
explained that it is and he means something else and this is not not
coming. I also suggested between the lines that he might try to move
the reader side to RCU.

John Ogness

Sebastian